This invention relates to the precise charging of secondary electrochemical cells and particularly to the relative rapid charging of cells without the need for supervision.
It has long been recognized that a desirable battery charger would be one which would bring the battery cells to be charged to a fully charged condition in a relatively short period of time without the need of supervision. A further desirable characteristic would be to enable the user to leave the battery connected to the charging apparatus without fear of damage to the cell even for prolonged periods. While conventional battery chargers have the capability of bringing the voltage of the battery up to a desired level in a relatively short period of time, oftentimes the charger then maintains the cell at this constant voltage level with the result that the cell will eventually be destroyed through overcharging. To compensate for this drawback, the charge voltage level has often been reduced which necessitates much longer charging periods in order to attain the desired capacity.
Some prior art investigators have advantageously employed the switching properties of a silicon controlled rectifier (SCR herein) to sense the battery cell voltage level and shunt the charging current away from the battery when the battery has attained its end of charge voltage. While this type of curcuitry has been generally satisfactory for the purposes to which it has been put, the characteristics of the individual circuits have been such that the shunt SCR does not always fire, or fires relatively late in A-C cycle (in response to the natural reduction in voltage of the cell which reduces the gate signal applied to the SCR). The result is that the charging circuit periodically returns to admitting high rate, quite often damaging, charging pulses to the battery cell. This type of circuitry has not always provided the precision required for many applications.
Pertinent prior art may be found in the United States Patent Office Classification Class 320, Electricity, Battery, and Condenser Charging and Discharging. Examples of the state of the art include U.S. Pats. Nos. 3,159,755; 3,278,823; 3,417,308; 3,436,639; and 3,531,706.
It is principal object of the present invention to provide a battery charging circuit capable of rapidly elevating a battery to its fully charged condition and then automatically terminating charging to prevent cell impairment.
It is another object of the invention to provide a charging circuit which utilizes a shunting circuit including a controlled rectifier in which the controlled rectifier is allowed to increase in temperature and respond to the temperature to provide a precise means for safely terminating charge.
It is still another object to eliminate high power dissipation losses during and after cutback of charging.
It is another object of the invention to provide a two-step charging rate for rapidly bringing the battery to be charged to its nearly full charged condition, followed by application of a predetermined trickle maintenance charge.
It is still a further object of this invention to provide electronic circuitry for a battery charger utilizing a minimum number of components and representing a low cost investment.
These and other objects are met and the disadvantages of the prior art solved by employing the method and construction of the present invention as described herein.